In a microfilm image processing apparatus of the type to which the present invention generally appertains, a frame of image recorded on a microfilm strip is picked up by a beam of light passed through or reflected from the microfilm strip. The resultant image-bearing beam of light is projected to an enlarged scale on a viewing screen by means of a projection lens. Examples of such a microfilm image processing apparatus include a microfilm image reader apparatus which enables the user of the apparatus to view the image reproduced on a viewing screen and an apparatus which not only reproduces the detected image on a viewing screen but produces electric signals responsive to the image-bearing beam of light with use of, for example, an array of charge-coupled devices. Typical of the apparatus of the latter type is a combination microfilm reader/printer apparatus which is capable of not only reproducing the detected image on a viewing screen but producing a printed output of the image detected from the microfilm strip.
To pick up an image recorded on a microfilm strip in a microfilm image processing apparatus of any of these types, the microfilm strip is irradiated with a beam of light emanating from a suitable form of light source. The resultant image-bearing beam of light is projected to an enlarged scale on a viewing screen by means of a projection lens located intermediate between the microfilm strip and the viewing screen. If the microfilm strip and the projection lens are spaced apart a fixed distance from each other, the projection lens will have its focal plane fixedly located with respect to the microfilm strip and accordingly to a target plane defined by the viewing screen. In actuality, however, the distance between the microfilm strip and the projection lens is inevitably subject to variation due to, for example, localized deformation of the microfilm strip and because of the fact that the holder plates holding the microfilm strip in place with respect to the projection lens tend to incline from the plane perpendicular to the path of light. A change thus caused in the distance between the microfilm strip and the projection lens results in dislocation of the focal plane of the lens and accordingly in inability of the lens to produce a correctly focused image on the viewing screen.
In order that the image to be reproduced on the viewing screen is correctly focussed, the projection lens may be moved with respect to the microfilm strip by having recourse to manipulative steps. For this purpose, the operator of the apparatus will be required to control the position of the projection lens by manually operating a focus control switch or switches while carefully viewing the image reproduced on the viewing screen. Extreme carefulness and highly skilled techniques as well as time-consuming manipulative efforts are necessitated for such manual control of the focal plane of the projection lens. Attempts have therefore been made to develop a microfilm image processing apparatus featuring automatic focus control capabilities. One of such attempts has resulted in a microfilm image processing apparatus disclosed in Japanese Provisional Patent Publication (Kokai) 62-105574.
In an automatic mode of focus control operation performed in a known microfilm image processing apparatus, the projection lens is automatically moved on the basis of a signal indicating a detected difference in contrast or a detected phase difference of the image to be reproduced. Where an image recorded on a microfilm strip has no contrast, such a signal could not be produced and accordingly the automatic mode of focus control is inoperable. In order to clearly reproduce such an image on a viewing screen, the operator of the apparatus is required to manually control the focal plane of the lens.
When the automatic mode of focus control is inoperable in the prior-art microfilm image processing apparatus taught in the named Publication, the automatic focus control operation is interrupted with the projection lens (or more exactly the focusing lens element forming part of a projection lens unit) held in the position to which the lens has been moved through execution of the automatic focus control operation. The operator of the apparatus attempting to manually control the focal plane of the projection lens is for this reason not informed of the current location of the focal plane of the lens and could not determine the direction in which the projection lens is to be moved. The operator must control the focal plane of the lens simply by a rule of thumb that requires time-consuming and nerve-racking manipulative efforts.
The present invention contemplates elimination of such a problem and, accordingly, aims at provision of a microfilm image processing apparatus having automatic focus control capabilities enabling the operator of the apparatus to easily and accurately control the focal plane of the projection lens when the automatic mode of focus control is inoperable.